It's a bit odd - I wouldn't be too perturbed (or surprised) if I picked up my kids from MIL to find out she had bought them new ones because she wasn't sure about the fit of the ones the child had on, and she'd do it in a slightly passive-aggressive implying I'm not paying attention to things way. But maybe that's just her.

It would annoy me too but I’d not mention it to her or discuss it in any way. Because if she was doing it to annoy me, I wouldn’t want to reward her behaviour.

If I didnt like the shoes, I’d give them to the charity shop and never mention it again. In the event that she asked about the shoes I’d just say airily “ oh yes hes grown out them now “ and change the subject.

If she is buying, suck it up and use as nursery/playing shoes. If not, get DH to tell her he doesn't want her to buy shoes or see if you can get a credit note without receipt.

Does my nut in that mothers/mils 'gifts' come with conditions. I am still smarting at my mother insisting she has all clothes bought for DD back after they have beeb outgrown so she can give then to her neice's child. She cries if I don't offer all clothes but I sold the last lot on ebay. It was worth it.